1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to improving handover performance.
2. Introduction
A wireless communication network may be deployed over a defined geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, macro access points (e.g., corresponding to different cells) are distributed throughout a macro network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the macro network. A macro network deployment is carefully planned, designed and implemented to offer good coverage over the geographical region. Such a careful planning cannot, however, completely accommodate channel characteristics such as path loss, fading, multipath, shadowing, and so on, in indoor environments. Indoor users, therefore, often face coverage issues (e.g., call outages, quality degradation) resulting in poor user experiences.
To supplement conventional network access points (e.g., macro access points), small-coverage access points may be deployed (e.g., installed in a user's home) to provide more robust indoor wireless coverage or other coverage for access terminals. Such small-coverage access points may be referred to as, for example, femto access points, femto cells, home NodeBs, home eNodeBs, or access point base stations. Typically, such small-coverage access points are connected to the Internet and the mobile operator's network via a DSL router or a cable modem. For convenience, small-coverage access points may be referred to as femto cells or femto access points in the discussion that follows.
An unplanned deployment of large numbers of femto cells may present various operational issues. As one example, issues may arise for in-bound idle mobility management of an access terminal from a macro network to a femto cell.
To facilitate in-bound idle mobility, an access terminal conducts searches for signals from nearby access points in an attempt to ensure, for example, that the access terminal will be served by the “best” available access point in that area. For example, as an access terminal roams throughout the geographical area associated with a network, the access terminal may move away from its serving access point. Consequently, signal conditions for the access terminal within a given cell may deteriorate, whereby the access terminal may be better served by another access point in the network. That is, it may be desirable for the access terminal to reselect to another cell (e.g., access point). A typical example would be where a mobile subscriber currently served by a macro cell comes to a location (e.g., the subscriber's home) where a femto cell for that subscriber is deployed.
To ensure that the “best” handover candidate may be readily identified when signal conditions at the current cell deteriorate, an access terminal regularly monitors for signals (e.g., beacon/pilot signals) from nearby access points to identify potential target access points to which the access terminal may be handed-over. In some cases, these access points may operate on a different carrier frequency than the current serving access point. Thus, this search may involve searching on different frequencies (i.e., an inter-frequency search). In some cases (e.g., where the access terminal has a single radio), the access terminal may tune away from its current carrier frequency to conduct such a search. Consequently, the access terminal may miss transmissions from its current serving cell during this search. For these and/or other reasons, a cell may specify certain parameters that are used to control how aggressively (e.g., under what signal conditions) an access terminal performs inter-frequency searches.
Depending on the current macro parameter settings, idle mode mobility management procedures specified in the 3GPP standard may not be triggered at an access terminal that is in the vicinity of a femto cell. As a consequence, an access terminal on a macro network may not be able to discover femto cells on other frequencies.